Boulder, Colo., USA: In this new article for Geology, Gavin Kenny and colleagues reveal the likely origin of Earth's oldest crystals. New research into the origin of Earth's oldest crystals suggests that they probably formed in huge impact craters rather than during the collision of tectonic plates moving around on Earth's surface as had previously been thought.
With very few rocks preserved from Earth's early history, the only material geoscientists have from this time comes in the form of tiny, naturally occurring crystals known as zircons. Naturally then, the origin of these crystals, which are approximately the width of a human hair, are more than four billion years old, and have famously suggested the presence of water on the very early Earth, has become a matter of major debate.
In this latest breakthrough, scientists from Trinity College, Dublin, and the Swedish Museum of Natural History, Stockholm, have shown that zircon crystals that formed in a much younger impact crater are indistinguishable from the very ancient zircons from early Earth. Given the fact that our planet suffered more asteroid impacts early on than it has in relatively recent times, this strongly suggests that many of the oldest crystals known to man could have formed in violent impact crater settings.
Differentiated impact melt sheets may be a potential source of Hadean detrital zircon
Gavin G. Kenny et al., Dept. of Geology, School of Natural Sciences, Trinity College Dublin, Dublin 2, Ireland. This article is online at http://geology.gsapubs.org/content/early/2016/04/27/G37898.1.abstract.
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Magma reservoirs from the upper crust to the Moho inferred from high-resolution Vp and Vs models beneath Mount St. Helens, Washington State, USA
Eric Kiser et al., Dept. of Earth Science, Rice University, 6100 Main Street, MS-126, Houston, Texas 77005, USA. This article is online at http://geology.gsapubs.org/content/early/2016/04/27/G37591.1.abstract.
This study uses seismic waves recorded during a temporary deployment of ~3500 seismographs to ~6000 locations to model seismic velocities beneath Mount St. Helens (MSH), currently the most active volcano along the Cascade Range. Anomalies in the velocities of P-waves (Vp) and S-waves (Vs), as well as the ratio of these velocities (Vp/Vs), allow us to infer the rock and fluid properties in the crust in the region surrounding this volcano. Based upon these anomalies and local seismicity, we infer the location of the primary magma reservoir (a region of small melt fraction) that feeds MSH, as well as a deeper zone southeast of MSH where magmatic fluids migrate through the lower and middle crust. Bounding this deep zone of magmatic fluids are two regions of high Vp in the lower crust inferred to be cumulates that have differentiated from magmas as they evolve after entering the crust. The spatial relationships between the inferred deep and shallow zones of magmatic fluids, seismicity, and cumulate bodies suggest that the boundaries of these cumulates may play an important role in both the vertical and lateral transport of magmas through the crust.
Iron-rich melts, magmatic magnetite, and superheated hydrothermal systems: The El Laco deposit, Chile
Fernando Tornos et al., Centro de Astrobiología (CSIC-INTA). Ctra. de Torrejón a Ajalvir, km 4.5, 28850 Torrejón de Ardoz, Madrid, Spain.
This article is online at http://geology.gsapubs.org/content/early/2016/04/27/G37705.1.abstract.
The El Laco volcano located in the Central Andes of northern Chile hosts one of the most enigmatic ore deposits in the world. The andesite lava flows host interbedded large bodies of massive magnetite that have been interpreted as lava flows. The origin of this mineralization has been the subject of major discussion, and both magmatic and hydrothermal models have been proposed to explain this unique mineralization and the related hydrothermal alteration. The hydrothermal system at El Laco magmatic-hydrothermal system has a deep zone of alkali-calcic alteration that is capped by a large zone of steam-heated alteration. In this contribution we present an alternative explanation to earlier models in which the iron mineralization and the related barren hydrothermal alteration are coetaneous and part of a protracted high-temperature magmatic-hydrothermal system that formed during the eruption of a water-rich iron-rich melt. Separation of water from the crystallizing iron-rich silicate-poor melt formed a unique superheated hydrothermal system that is perhaps the hottest recorded on Earth. This is attributed as being due to its relationship with this iron-rich melt, which has a solidus temperature well above that of the host andesite.
Partial resetting of the U-Th-Pb systems in experimentally altered monazite: Nanoscale evidence of incomplete replacement
Alexis Grand'Homme et al., ISTerre (Institut des Sciences de la Terre), University Grenoble Alpes, 38058 Grenoble Cedex 9, France. This article is online at http://geology.gsapubs.org/content/early/2016/04/27/G37770.1.abstract.
With the development of in situ dating techniques, it is now possible to distinguish several growth episodes within a crystal. For example, the existence of the oldest terrestrial material has been revealed thanks to in-situ dating in micro-domains of zircon crystals from Jack Hills (Australia). Yet we demonstrate that micrometer scale, currently reached for in-situ dating, may be insufficient regarding the spatial scale of the mineralogical processes involved during fluid-mineral interactions. The present experimental study focuses on alteration of monazite crystals (555 Ma) under alkali conditions in the range of 300-600 °C/200 MPa. Above 400 °C, experimental products show typical of natural replacement textures: the primary monazite (Mnz1) is partially altered forming a porous rim with different composition. In situ U-Th-Pb dating in the altered rim indicates that the U-Th-Pb ages have been only partially reset during experimental alteration (intermediate ages between 555 Ma and 0 Ma). Thanks to Transmission Electron Microscope images, mechanisms behind incomplete resetting is clarified: Mnz1 is incompletely replaced in the altered rim by a secondary monazite (Mnz2), free of Pb. The presence of nano-mixture of inherited and secondary monazite in replaced domains accounts for possible age disturbance in complexly zoned grains.
Multiple thermo-erosional episodes during the past six millennia: Implications for the response of Arctic permafrost to climate change
Melissa L. Chipman et al., Program in Ecology, Evolution, and Conservation Biology, University of Illinois, 505 S. Goodwin, Urbana, Illinois 61801, USA. This article is online at http://geology.gsapubs.org/content/early/2016/04/27/G37693.1.abstract.
Anthropogenic climate warming is amplified in the Arctic. Projected warming in the 21st century may promote permafrost degradation, with far-reaching implications for the fate of the large carbon stocks in frozen soils. Thermo-erosion represents a key pathway for rapid carbon releases from ice-rich terrains, yet the long-term variability and drivers of thermo-erosional activity remain unclear. Chipman et al. use geochemical signals of lake sediments to reconstruct 6000 years of thaw slump activity on Alaska's North Slope. They find strong evidence for at least 10 episodes of permafrost thaw and associated erosion. These episodes generally coincided with periods of elevated summer temperature. Positive feedbacks appear to have facilitated the repeated activation of thermo-erosion. These results highlight the sensitivity of permafrost terrain to climate warming and the unstable nature of Arctic landscapes.
Seismic interpretation of crustal-scale extension in the Intermontane Belt of the northern Canadian Cordillera
Andrew J. Calvert, Dept. of Earth Sciences, Simon Fraser University, Burnaby, British Columbia V5A 1S, Canada. This article is online at http://geology.gsapubs.org/content/early/2016/04/27/G37767.1.abstract.
The North American Cordillera is one of Earth's great mountain belts, and was created by the subduction-driven collision of offshore island chains against the west coast, a process which also thickened the North American crust. At its maximum during the Cretaceous, the crustal thickness probably reached 50-65 km, but this has since fallen to 32-38 km as the mountain belt collapsed and the underlying crust extended. In southern British Columbia, geological and geophysical evidence of this extension is well accepted, but in the Yukon and northern British Columbia there is little evidence of extension despite the well determined lower crustal thickness. Using seismic reflection images acquired by the Canadian Lithoprobe program, extension is inferred from a major fault that extends from close to the surface to 20 km depth, the geometry of reflections in the overlying structural basin, and a lateral change in seismic velocity. The identified 10-km-deep structural basin appears to be a significant part of the northern Canadian Cordillera in Yukon and northern British Columbia.
Identifying ideal stratigraphic cycles using a quantitative optimization method
Peter M. Burgess, School of Environmental Sciences, University of Liverpool, Jane Herdman Building, Liverpool L69 3GP, UK. This article is online at http://geology.gsapubs.org/content/early/2016/04/27/G37827.1.abstract.
The ideal cycle concept is poorly defined yet implicit and potentially useful in many stratigraphic analyses. A new method allows quantitative definition of ideal cycles and provides a simple but robust method to analyze stratal order and quantify stratigraphic interpretations. The method calculates transition probability (TP) matrices from a vertical succession of strata for all possible permutations of facies-class row numbering in the matrices. The ordering of facies classes that gives highest transition probabilities along diagonals of the TP matrix can be taken as a quantitative definition of an ideal cycle for the strata being analyzed. Application to a synthetic example shows how an ideal cycle can be identified, even in noisy strata, without any assumptions about or prior knowledge of cyclicity. Application to two outcrop examples shows how it can be useful to define the most optimal cycle and determine how much evidence is present for ordered and cyclical facies successions.